1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus using a hydraulic cylinder as a source for generating a force assisting a steering force, more particularly, it relates to a power steering apparatus including a constraining device which constrains switching operation of a directional control valve for switching the feeding direction of pressured oil to the hydraulic cylinder corresponding to the vehicle speed.
2. Description of the Prior Art
A hydraulic power steering apparatus designed to generate a force assisting a steering force by a hydraulic cylinder disposed in a steering mechanism and to reduce a force required for steering operation, is so constructed that, the feeding direction of pressured oil to two oil chambers in the hydraulic cylinder are switched by a directional control valve disposed between the hydraulic cylinder and a hydraulic pressure source according to the operating direction of a steering wheel.
In the directional control valve, an input shaft which rotates interlockingly with the steering wheel and an output shaft which rotate interlockingly with operation of the steering mechanism are connected coaxially through a tortion bar, and a plurality of elongated grooves extending axially are formed on the peripheral surface near the connecting end of the input shaft to constitute a valve body. While, a cylindrical casing provided with the same number of elongated grooves as the aforesaid elongated grooves, extending axially on its inner circumferential surface is secured to the connecting end of the output shaft coaxially. The valve body is inserted rotatably into the casing so as to be positioned such that their elongated grooves are arranged in a zigzag fashion. The elongated grooves of the casing are brought in communication with two oil chambers in the hydraulic cylinder alternately, and the elongated grooves of the valve body are brought in communication with the oil pressure source and oil tank alternately. Corresponding to distortion produced on the torsion bar according to the steering wheel operation, the relative angular displacement is produced between the valve body and the casing. At this time, gap areas between the elongated groove of the valve body which is in communication with the hydraulic pressure source, and the elongated grooves of the casing adjacent to elongated groove of the valve body become larger on one side and smaller on the other side, so that a pressure difference is developed between two elongated grooves of the casing and between two oil chambers of the hydraulic cylinder which are respectively in communication with the former. By this pressure difference, the hydraulic cylinder is operated to generate a force assisting the steering force in the direction corresponding to the operating direction of the steering wheel.
Now, a force required for steering operation at manual steering corresponds to a resistance force from the road surface acting on wheels, and a large operating force is necessary at low speed driving and standstill. While, at high speed driving, the steering operation can be effected by a small operating force. Thus, in a power steering apparatus, characteristics which generates as large force assisting the steering force as possible is required to reduce the force required for steering operation at low speed driving and standstill. On the other hand, at high speed driving, characteristics which hardly generates any force assisting the steering force is required to provide appropriate rigidity to the steering wheel and to improve the rectilinear stability. However, in the power steering apparatus having the aforesaid construction, correlation between the operating force acting on the steering wheel and the force assisting the steering force produced by the hydraulic cylinder is subject to the correlation between the operating force acting on the vehicle is steered a torsional angle produced on the torsion bar thereby. When selecting torsional characteristics of the torsion bar to realize the characteristics aforementioned at the low speed driving and standstill, vehicle is steered by a small operating force applied to the steering wheel at high speed driving and the rectilinear stability is deteriorated. While, when determining the distortional characteristics to realize the characteristics aforementioned at high speed driving, there is a difficulty in obtaining the sufficient assisting steering force at low speed driving and standstill.
In order to solve the difficulty, a power steering apparatus using a torsion bar having torsional characteristics whereby the sufficient assisting steering force is obtained at low speed driving and standstill, and at the same time, provided with a constraining device (hydraulic reaction force device) between the valve body and the casing of the directional control valve for applying a constraining force corresponding to the vehicle speed as disclosed, for example, in Japanese Patent Application Laid-Open No. 61-200063 (1986), has been proposed. By constraining the aforesaid relative angular displacement by the constraining device to make switching operation of the directional control valve difficult at high speed driving, two contradictory characteristics may be realized.
In the constraining device, to the cylindrical portion formed by extending end portions of the casing, a plurality of plungers are mounted slidably and radially with their tips directing inwardly. The tips of the plungers are pressed against the peripheral surface of the valve body by oil pressure introduced into the base portion side of the plungers so as to apply the constraining force.
Into the constraining device, successively through a variable throttle whose throttle opening is adjusted proportional to the vehicle speed detected by a speed sensor and a fixed throttle having a fixed throttle opening, oil pressure between the variable throttle and the fixed throttle in an oil pressure circuit extending from the oil pressure source to the oil tank, which changes proportional to the vehicle speed is introduced, thereby the constraining force corresponding to the vehicle speed is obtained.
However, in the conventional power steering apparatus having the constraining device thus constructed, there is a possibility that a control portion which adjusts the throttle opening of the variable throttle in response to the output signal from the speed sensor is subjected to malfunction by reacting to the external disturbance signals such as electromagnetic waves and produces an error control signal. By the malfunction, the throttle opening of the variable throttle which is adjustable responsive to the output signal from the speed sensor, or the constraining force applied by the constraining device does not respond to the vehicle speed, thereby not only the rigidity of the steering wheel is changed suddenly and the steering sensation is worsened, but if the steering wheel becomes lighter suddenly during high speed driving, there has been the possibility of incurring an unexpected accident.